I'll pretty much guarantee 20:1 alloy won't shoot worth a dern in that rifle - go with pure lead. Slug your bore and mic/caliper the slugto find out the correct sizing. Recommend just a hair bigger than measured land to land dimensions. Dave's rifle very probably has a different bore dimension, TC barrel tolerancescan result in some significant differences. You could specify your exact land to land but ask for the single step sizing which will result in a slight increase. Recommend his #2 lube. Understand he can also produce some .50 cal conicals in the range of 300 to 350 grains, you might ask about those as they might perform better in that rifle.